The invention relates to a brake-steering system, particularly for off-road vehicles, for example, tractors for agriculture or forestry or an excavator-loader from the field of construction machines, wherein the steering of the vehicle is aided by actuation of brakes on either side of the vehicle.
Brake-steering systems, which most often operate a hydraulically are used conventionally to aid in steering vehicle by selective braking of the ground support elements located on the side of the vehicle facing the center of curvature of the turn being made, i.e., the curve-internal side of the vehicle. In this connection, the procedure is already known of providing for each side of the vehicle a separate main brake cylinder with which the left and right brake circuits can be connected separately. To reduce the investment of technical devices, it has been proposed to couple a common main brake cylinder with a hand-actuated reversing valve, an approach (depending on the switch position) with which either braking on one side of the vehicle or braking on both sides can be achieved.
However, because hand-switching can impair driving safety, a brake-steering system has become known in accordance with the disclosure of German Patent No. DE-AS 14 80 204, which calls for double pedal comprised of two half-pedals for the left and right sides of the vehicle. When one half-pedal is actuated, only one side of the vehicle is braked, but by pressing on both half-pedals both sides of the vehicle can be braked, i.e., dual-sided braking. The valves of the reversing-valve device for impacting the left or right wheel-brake cylinder in this known case are constructed as seat valves, which are held in closed position, against the brake pressure delivered from the main brake cylinder, the holding force being established, by the restoring force of the double pedal. If one half-pedal is pressed, then the valve body of the associated reversing valve is lifted off the valve seat, so that the brake pressure delivered from the main brake cylinder is conducted to the associated wheel-brake cylinder in order to initiate brake-steering.
Apart from the fact that this known brake-steering system of German Patent No. 14 80 204 requires relatively space-consuming tubing arrangements in the vehicle, it has been found that this type of brake-steering cannot cope with certain critical driving situations, particularly in difficult terrain. This can be traced to the fact that with such a system a reduction of pressure on the wheel-brake cylinder on only one side of the vehicle cannot be achieved directly from the dual-sided braking operation. That is, a brake-steering phase cannot be entered directly from a dual-sided braking phase. Rather, the dual-sided braking process must first be cut off, i.e., the brake must be fully released, before the operator can actuate a single half-pedal to initiate a brake-steering operation.
Such a procedure runs counter to the natural reaction patterns of the vehicle driver. Particular in off-road operation, the vehicle must often be braked and steered at the same time. In this already critical driving situation, the driver can therefore consider releasing the brake only if the natural reaction pattern is nullified by way of a mental reasoning process. This means that hitherto in such critical driving situations, the driver had to do without the aid of brake-steering. Even if the driver employed brake-steering by a repeated releasing of the brakes, such a procedure resulted in a loss of time, which in particularly critical driving situations can impair safety.
One purpose of the invention, therefore, is to create a fluid-operated brake-steering system which occupies minimal space, and which can be safely operated in all driving situations that the vehicle encounters.